Intrusion detection systems are known for protecting a plurality of zones within a facility. Such systems usually include a central control unit connected by means of a cable to one or more sensors located at each protected zone. Upon an alarm condition in a zone, an alarm signal is transmitted back to the control unit for actuation of an alarm and desirably to denote which zone in which an alarm has occurred. It is often desirable to provide an intrusion detection system in which different types of sensors are employed in respective zones. For example, it may be useful to employ an ultrasonic sensor in one zone and an active or passive infrared sensor in another zone to suit specific performance requirements. It would be advantageous to accommodate sensors of different types in a single system without need for alteration of the system itself to permit connection of different types of sensors thereto.